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The QCL is based on a mechanism (inter sub-band transitions) that exhibits a poor efficiency: most of the electrons emit phonons instead of photons. The laser thus heats a lot and thermal management at the microscopic scale of the waveguide is important to allow operation. The waveguide is thus very small in the vertical direction (i.e. perpendicular to the quantum wells plane) in order to optimize the overlap between the optical mode and the gain region.